A head mounted display (HMD) is a display device worn on or about the head of a user. HMDs usually incorporate some sort of near-to-eye optical system to display an image within a few centimeters of the human eye. Single eye displays are referred to as monocular HMDs while dual eye displays are referred to as binocular HMDs. Some HMDs display only a computer generated image (CGI), while other types of HMDs are capable of superimposing CGI over a real-world view. The former type of HMD is often referred to as virtual reality (VR) while latter type of HMD is often referred to as augmented reality (AR) because the viewer's image of the world is augmented with an overlaying CGI, also referred to as a heads-up display (HUD).
As HMDs incorporate an increasing amount of computing capacity and input/output means (e.g., audio data output, audio/image data capture), they also potentially consume more power. It is important for an HMD to be aware of how it is being used in order to efficiently activate or de-activate its components.